Personal computers and workstations have been widespread as image processing apparatuses that process image data. Application software that runs on the above image processing apparatuses forms image data formed of various objects (character, painting, line, and photograph).
There exists printers, facsimile machines, copiers, and multifunction peripherals having functions of the printers, the facsimile machines, and the copiers, as image forming apparatuses that form and output an image of the image data. As for an image forming method, there exists for example an inkjet recording method and electrophotography, and form an image by using image forming material such as recording liquid (ink) or toner.
Among the above image forming apparatuses, apparatuses that perform digital image recording by using the inkjet system is increasingly developed and becoming popular.
In general, an inkjet recording apparatus includes a recording means (print head), a carriage for mounting an ink tank, a conveying means for conveying a recording sheet, and a control means for controlling these means. In recent years, a so-called serial system is becoming popular as the inkjet recording system.
In the serial system, a print head that ejects ink droplets via a plurality of ejection ports is caused to perform serial scanning in a direction (main-scanning direction) perpendicular to conveying direction of a recording-sheet (sub-scanning direction), and intermittent conveyance is performed by the amount equal to a recording width when recording is not performed.
Color inkjet recording apparatuses form color images by layering ink droplets ejected from print heads for a plurality of colors.
Meanwhile, there has been proposed a so-called bidirectional printing technology for ejecting ink droplets during scanning in forward and backward direction in the main-scanning direction in order to increase image forming speed. Furthermore, many inkjet recording apparatuses have a monochrome printing mode for saving ink. Moreover, there is a demand for decreasing the number of print heads as one way of reducing costs of inkjet recording apparatuses. It is desired to achieve high speed and high image quality while meeting the above three requirements. Problems in achieving high speed and high image quality will be explained below.
High Image Quality in Bidirectional Printing
As a problem specific to bidirectional printing, band unevenness that occurs in the main-scanning direction has been known (hereinafter, referred to as a bidirectional color difference). The bidirectional color difference is a phenomenon that occurs due to a significant color difference caused by change between order of layering of colors during the scanning in the forward direction and order of layering of ink during the scanning in the backward direction.
For example, when red is created, the color is produced by mixing magenta ink and yellow ink. However, when magenta ink is first ejected and then yellow ink is ejected on the magenta ink, magenta-like red is created. When ejection is performed in reverse order, yellowish red is created. That is, firstly-ejected color becomes dominant.
Unlike dye ink that is dissolved in ink, pigment ink or colored-resin-emulsion-containing ink, in which particulate colorant components are dispersed, is greatly affected by the order of layering. Therefore, the above phenomenon is an extremely big problem. The problem with the bidirectional color difference can be solved by unifying the order of layering of ink in both directions.
High Speed in Monochrome Printing
In a serial scanning system, there is a known method for increasing a head width in order to achieve high speed. This allows increase in a double-wide image formation width, so that image formation can be complete at higher speed. With use of this method, it is possible to speed up monochrome printing. More specifically, various methods have been proposed, in which, for example, a long head is provided for specific ink or the number of ejection nozzles for specific ink is increased.
To solve the problem with the high speed and high image quality as described above, Japanese Patent Application Laid-open No. 2004-106392, for example, discloses a technology for disposing a color nozzle in a nozzle row direction and unifying bidirectional landing order of color ink in order to reduce a bidirectional color difference. In the method disclosed in Japanese Patent Application Laid-open No. 2004-106392, speed of black monochrome printing can be increased by providing a black nozzle separately from a color nozzle.
Japanese Patent No. 4144852 discloses a technology for increasing the length of a black nozzle to more than double the length of a color nozzle in order to unify the bidirectional landing order and increase the speed of black monochrome printing.
Japanese Patent Application Laid-open No. 2001-171151 discloses a technology for switching between a head used in a main scanning in a forward direction and a head used in the main scanning in a backward direction in order to unify the bidirectional landing order.
Japanese Patent Application Laid-open No. 2005-305959 discloses a technology for symmetrically disposing color nozzles in the sub-scanning direction in order to unify the bidirectional landing order and to increase the speed of image formation.
However, in the method disclosed in Japanese Patent Application Laid-open No. 2004-106392, there is a problem in that it is difficult to unify the landing order for nozzles including the separately-disposed black nozzle. In the methods disclosed in other Patent Literatures, there is a problem in that a head having a specific structure or a plurality of heads is needed.
The present invention has been made in view of the above, and it is an object of the present invention to provide an image forming apparatus and a control method capable of unifying the landing order in bidirectional printing and increasing the speed of monochrome printing.